Fish oil explained: what it is, how it works and who it's for

What fish oil is, and what is actually in it

Fish oil is oil taken from fatty fish and sold mostly for two fats: EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). Both belong to the omega-3 family. "Omega-3" is a category of fats, not a single ingredient, and fish oil is one marine source of it. Our fish oil topic page collects the rest of our coverage.

Plants supply a third omega-3, ALA, found in flaxseed, chia, walnuts and canola oil. The body can convert some ALA into EPA and DHA, but only in small amounts. ALA-rich foods are therefore not a direct substitute for the EPA and DHA in fish or fish oil.

Food is the baseline for comparison. Per USDA data, a 3-ounce cooked serving of mackerel provides roughly 1,800–2,600 mg of combined EPA and DHA, and salmon roughly 1,000–1,900 mg. Canned light tuna and shrimp land around 150–300 mg. A supplement's main selling point is consistency: a fixed, known amount each day instead of a variable fish serving.

Blends sold as omega 3-6-9 are a different product. Omega-6 and omega-9 fats are structurally distinct, and most Americans already get plenty of omega-6 from vegetable oils. A fish oil concentrate is built around EPA and DHA alone.

How it works in the body

When you take EPA and DHA, they show up in your blood and in cell membranes. Researchers track this with the omega-3 index, a lab test of how much EPA and DHA sits in red blood cell membranes. In a trial of 115 healthy adults, daily doses from 300 mg to 1,800 mg of EPA and DHA raised the index in a dose-dependent way over about five months.

That result is a biomarker, not a health outcome. It tells you the supplement does what it is supposed to do at the level of blood chemistry, and that more generally means a higher level. It does not tell you that a higher index makes you healthier.

The index has been proposed as a cardiovascular risk marker, with 8% or higher linked to the lowest risk and 4% or lower to the highest in the studies reviewed. Treat that as a research risk score, not a clinical deficiency diagnosis.

Marine omega-3 also shows up in inflammation research. A 2014 meta-analysis of 68 trials found it lowered the lab markers CRP, IL-6 and TNF-alpha, with larger effects from longer supplementation. Lower markers are not the same as feeling better, and as the table below shows, symptom trials are far less consistent.

The heart: where the evidence is strongest, and where it frays

Triglycerides are the clearest case. A 2019 American Heart Association advisory concluded that prescription omega-3 at 4 g a day lowers triglycerides by at least 30% in adults who already have high levels. That is a prescription dose, several times what a typical over-the-counter softgel provides.

Hard outcomes are messier. One trial in 8,179 people already treated for cardiovascular disease or diabetes found that 4 g a day of a prescription EPA drug cut a composite of cardiovascular events from 22.0% to 17.2% over about five years, but also increased hospitalization for atrial fibrillation. An older trial in 11,324 heart attack survivors found a 10–15% reduction in a composite of death, non-fatal heart attack and stroke with 1 g a day.

Then comes the pooled picture. The largest review to date, covering 86 trials and 162,796 people, found little or no effect of omega-3 on death, cardiovascular events, stroke or arrhythmia at any dose tested. A separate 2021 meta-analysis of seven large trials linked marine omega-3 to a higher risk of atrial fibrillation, with the risk rising at doses above 1 g a day.

Blood pressure shows a modest effect. A dose-response analysis of 71 trials found reductions of about 2.6 mmHg systolic and 1.6–1.8 mmHg diastolic, concentrated at 2–3 g a day of EPA and DHA. An umbrella review across all doses put the average lower, about 1.2 and 0.9 mmHg. In people with type 2 diabetes, a Cochrane review found omega-3 lowered triglycerides but raised LDL cholesterol slightly, with no effect on blood sugar.

Everything else: a claim-by-claim reality check

Fish oil has been tested for a long list of conditions. The pattern is consistent: small effects, very low certainty, benefits that appear only at high doses, or nothing at all.

A few findings sit in the margins. Some trials of eczema and acne used small samples, and the eczema trial used 5.4 g of DHA a day, far beyond a typical dose. Most trials of omega-3 in these areas are too small or too short to settle anything. Omega-3 intake from diet also showed an association with lower success of H. pylori antibiotic treatment in an observational study, a reminder that more is not uniformly better.

Who it is for

The honest answer is narrower than the marketing. Fish oil makes the most sense for people who rarely eat fatty fish and want a predictable amount of EPA and DHA. It is not a stand-in for a health plan, and it does not treat or prevent disease.

• People who skip fish. A single serving of salmon or mackerel supplies several times the EPA and DHA of a typical softgel, so eating fish does the same job. A supplement fills the gap when you will not.
• People who avoid animal products. In a 2025 trial of 74 adults, algae-oil omega-3 raised blood DHA and EPA about as well as fish oil over 14 weeks.
• Adults with high triglycerides. The effective doses are prescription-level, so this is a conversation for a clinician, not a shelf purchase.
• Pregnancy. A Cochrane review of 70 trials found omega-3 lowered preterm birth rates from 13.4% to 11.9%, with a probable small increase in pregnancies lasting past term. Dose and product are worth settling with a prenatal provider.

If you want a broader checklist, the guide on who should not take fish oil covers the cautions in more depth.

Dose, form and how to take it

No single dose fits every goal. The omega-3 index rose even at 300 mg a day, but most trials that found clinical effects used 2 g or more. That gap matters: a result from a 2–4 g trial does not transfer to a 300 mg softgel. Check the label for EPA and DHA amounts, not just the total milligrams of oil.

Form is a live debate. Without food, free fatty acid fish oil was absorbed best in one small single-dose trial, the natural triglyceride form somewhat less well, and ethyl esters about a fifth as well as the free acid. But when doses were matched exactly over four weeks in 66 healthy adults, blood levels did not differ significantly between ethyl ester, triglyceride and krill oil. That trial was funded by an ingredient manufacturer. For more detail, see this comparison of fish oil forms.

Food is the easier lever. In a small 1988 trial, eating fish oil with a high-fat meal raised EPA absorption from the triglyceride form from 69% to 90% compared with a lower-fat meal, and roughly tripled absorption of both fats from the ethyl ester form. Taking it with a meal is a sensible default.

Safety, side effects and interactions

Side effects are mostly minor. A meta-analysis of 21 trials (24,460 people) using prescription-strength omega-3 found fishy aftertaste and skin reactions were more common than with placebo, and EPA plus DHA combinations caused more burping and nausea. No clear evidence of serious adverse events turned up.

Two cautions deserve attention. First, fish oil has a weak blood-thinning effect on its own, which becomes more noticeable alongside antiplatelet drugs and, to a lesser extent, warfarin or factor Xa inhibitors. Alone, it was not linked to a meaningful increase in surgical bleeding risk. Second, atrial fibrillation risk rose with dose in large trials, so higher intakes are not simply a stronger version of lower ones.